1. Field of the Invention The present-invention relates to a thermoelectric semiconductor material and a method for manufacturing same.
2. Description of the Related Art
A thermoelectric cooling element which uses Peltier effect or Ettingshausen effect or a thermoelectric power generation element which uses Seebeck effect is attracting attention in the point that it can be used over the wide range because it has a simple structure, is handled easily and keeps stable characteristics. Particularly, as a thermoelectric cooling element can control a local cooling and a precise temperature in the vicinity of room temperature, its extensive research and development are being made for its application to optoelectronics, temperature control of semiconductor lasers, a small refrigerator and the like.
A material of the thermoelectric element used for the thermoelectric refrigeration and thermoelectric generation electricity has a figure of merit Z(=xcex12/xcfx81xcexa) represented by Seebeck coefficient xcex1, electrical resistivity xcfx81 and thermal conductivity xcexa, which are peculiar constants of the material in a range of temperatures it is used.
Specifically, a crystal material generally used for thermoelectric cooling element and the like includes mixed crystal such as bismuth telluride (Bi2Te3), antimony telluride (Sb2Te3), and bismuth selenide (Bi2Se3).
Such Bi2Te3 based semiconductor materials, e.g., bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3), have a layer structure as shown in FIG. 10. Specifically, Te, Bi (or Sb), Te, Bi (or Sb) and Te are bonded sequentially from top to bottom into a five-layer form by covalent bonding. And, three of such five-layer forms are bonded into one 15-layer unit by Van der Waals bonding or ionic bonding so to form a stratified layer crystalline structure.
Meanwhile, a PbTe-based or PbSnTe-based semiconductor material is used as a thermoelectric conversion material used in a middle temperature range (from room temperature to 400xc2x0 C.).
Such PbTe-based and PbSnTe-based semiconductor materials have high mobility of a carrier in the crystal. This semiconductor material provides a remarkable thermoelectric property when it is in a monocrystalline state or formed into a polycrystalline ingot material by solidifying in one direction.
But, since its material strength is low for industrial use, it is necessary to improve the strength by forming into a sintered body.
However, the sintered body has an increased resistance and a lowered thermoelectric property because an influence of scattering in a grain field due to high carrier mobility.
As described above, the crystal material generally used for the thermoelectric cooling element or the like includes mixed crystals such as bismuth telluride (Bi2Te3), antimony telluride (Sb2Te3), and bismuth selenide (Bi2Se3).
Conventionally, a method for synthesizing a compound consisting of a plurality of elements as described above was a xe2x80x9cmelting and solidifying methodxe2x80x9d which seals raw material elements into a glass tube, melts them to synthesize in a liquid phase, and cools to solidify.
The above melting and solidifying method may not provide a uniform phase because two phases are formed depending on the composition of the compound during solidifying by simply cooling a liquid phase.
For example, it is assumed that compound PbBi2Te4 is synthesized.
The compound PbBi2Te4 is a compound having PbTe and Bi2Te3 mixed at a stoichiometric ratio of 1:1. The phase diagram of this PbTe-Bi2Te3 alloy is shown in FIG. 12. It is apparent from the drawing that when PbBi2Te4 is synthesized by the melting and solidifying method, a compound having a uniform structure cannot be obtained because it is separated into a compound which includes PbTe excessively than the target composition PbBi2Te4 and a compound which includes Bi2Te3 excessively than the target composition PbBi2Te4.
Specifically, a composition of 50 mol % Bi2Te3-50 mol % PbTe is indicated by straight line 1 in FIG. 12. The solid PbBi2Te4 compound is formed when a temperature is below horizontal line 5 on the straight line 1. Then, when a solute, which was prepared by weighing to have the aforesaid composition and dissolving, is cooled down to temperature T1 (melting point) at point A where the straight line 1 intersects with liquidus line 2, a solid of composition 4 corresponding to point B (temperature T1) on solidus line 3 is precipitated from the solute.
When the temperature is further lowered, the composition of the solute changes from point A to point C, and the composition of the precipitated solid changes from point B to point D. Upon reaching temperature T2 of the horizontal line 5, the solute of the composition at point C and the solid of the composition at point D are present at the same time. At temperature T2, the solute at point C reacts with the solid at point D. If temperature T2 can be maintained for a sufficient period of time, the solute at point C and the solid at point D are fully reacted, and the compound of composition PbBi2Te4 at point C can be formed uniformly. But, it is practically difficult to keep a fixed temperature for a very long period, and a compound having a different composition is precipitated due to a little change in temperature. As a result, a solid obtained by cooling contains the compound having the composition in the vicinity of point C and the compound of the composition in the vicinity of point D in addition to the desired PbBi2Te4 compound.
Accordingly, for such a compound which cannot be provided with a uniform phase by the melting and solidifying method, a zone melting method or a zone leveling method can be used.
But, an apparatus for conducting the zone melting method or the zone leveling method is complex and expensive. And, an obtained ingot often has a portion with a composition different from the target composition, resulting in a poor yield.
Meanwhile, a solid phase reaction which synthesizes at a temperature lower than the melting point may be applied in order to prevent the separation into two phases at the time of soldering.
But, a long period of tens of hours is necessary to fully complete the reaction by the solid phase reaction. Thus, a production efficiency becomes poor and a production cost is high.
The present invention was achieved in view of the aforesaid circumstances. And it is an object of the invention to provide a new thermoelectric semiconductor material with an improved thermoelectric property by having the similar uniform phase layer structure as shown in FIG. 10 even if the material is sintered as compared with a conventional PbTe-based or PbSnTe-based semiconductor material.
Accordingly, a thermoelectric semiconductor material according to a first aspect of the invention has chemical formula AB2X4 (where, A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance or mixture of Bi and Sb (V family elements), and x is a simple substance or mixture of Te and Se (VI family elements)).
A thermoelectric semiconductor material according to a second aspect of the invention has chemical formula Bi1xe2x88x92xSbxTe1 (where, 0xe2x89xa6xxe2x89xa61).
A thermoelectric semiconductor material according to a third aspect of the invention has chemical formula AyB1xe2x88x92yX1 (where, 0xe2x89xa6yxe2x89xa60.25, A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance or mixture of Bi and Sb (V family elements), and X is a simple substance or mixture of Te and Se (VI family elements)).
A method of producing a thermoelectric semiconductor material according to a fourth aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula AB2X4, where A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance or mixture of Bi and Sb (V family elements), and X is a simple substance or mixture of Te and Se (VI family elements), by applying a pulsed current to material powder for the thermoelectric semiconductor material having the A, B and X mixed in a stoichiometric ratio of 1:2:4 to cause an electrical discharge among particles of the powder.
A method of-producing a thermoelectric semiconductor material according to a fifth aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula Bi1xe2x88x92xSbxTe1 by applying a pulsed current to material powder for the thermoelectric semiconductor material having Bi, Sb and Te mixed in a stoichiometric ratio of 1xe2x88x92x:x:1 (where, 0xe2x89xa6xxe2x89xa61) to cause an electrical discharge among the particles of the powder.
A method of producing a thermoelectric semiconductor material according to a sixth aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula AyB1xe2x88x92yX1, where A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance or mixture of Bi and Sb (V family elements), and X is a simple substance or mixture of Te and Se (VI family elements), by applying a pulsed current to material powder for the thermoelectric semiconductor material having the A, B and X mixed in a stoichiometric ratio of y:1xe2x88x92y:1 (where, 0xe2x89xa6yxe2x89xa60.25) to cause an electrical discharge among particles of the powder.
A method of producing a thermoelectric semiconductor material according to a seventh aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula AB2X4 as well as forming a sintered body of the compound, where A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance of mixture of Bi and Sb (V family elements), and X is a simple substance or mixture of Te and Se (VI family elements), by applying a pulsed current and a pressure to material powder for the thermoelectric semiconductor material having A, B and X mixed in a stoichiometric ratio of 1:2:4 to cause an electrical discharge among particles of the powder.
A method of producing a thermoelectric semiconductor material according to an eighth aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula Bi1xe2x88x92xSbxTe1, as well as forming a sintered body of the compound by applying a pulsed current and a pressure to material powder for the thermoelectric semiconductor material having Bi, Sb and Te mixed in a stoichiometric ratio of 1xe2x88x92x:x:1 (where, 0xe2x89xa6xxe2x89xa61) to cause an electrical discharge among particles of the powder.
A method of producing a thermoelectric semiconductor material according to a ninth aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula AyB1xe2x88x92yX1 as well as forming a sintered body of the compound, where A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance or mixture of Bi and Sb (V family elements), and X is a simple substance or mixture of Te and Se (VI family elements), by applying a pulsed current and a pressure to material powder for the thermoelectric semiconductor material having A, B and X mixed in a stoichiometric ratio of y:1xe2x88x92y:1 (where, 0xe2x89xa6yxe2x89xa60.25) to cause an electrical discharge among particles of the powder.
A thermoelectric semiconductor material according to a sixteenth aspect-of the invention has chemical formula Bi1xe2x88x92xSbxTe1xe2x88x92ySey (where, 0xe2x89xa6xxe2x89xa61and 0xe2x89xa6y23 1).
A thermoelectric semiconductor material according to a seventeenth aspect of the invention has chemical formula Bi2xe2x88x92xSbxTe1xe2x88x92ySey (where, 0xe2x89xa6xxe2x89xa62 and 0xe2x89xa6yxe2x89xa61).
A thermoelectric semiconductor material according-to an eighteenth aspect of the invention has chemical formula (Bi1xe2x88x92xSbx)4(Te1xe2x88x92ySey)5 (where, 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61).
A method of producing a thermoelectric semiconductor material according to a nineteenth aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula Bi1xe2x88x92xSbxTe1xe2x88x92ySey by applying a pulsed current to material powder for the thermoelectric semiconductor material having Bi, Sb, Te and Se mixed in a stoichiometric ratio of 1xe2x88x92x:x:1xe2x88x92y:y (where, 0xxe2x89xa6xxe2x89xa61 and 0 less than yxe2x89xa61) to cause an electrical discharge among particles of the powder.
A method of producing a thermoelectric semiconductor material according to a twentieth aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula Bi2xe2x88x92xSbxTe1xe2x88x92ySey by applying a pulsed current to material powder for the thermoelectric semiconductor material having Bi, Sb, Te and Se mixed in a stoichiometric ratio of 2xe2x88x92x:x:1xe2x88x92y:y (where, 0xe2x89xa6x23 2 and 0xe2x89xa6yxe2x89xa61) to cause an electrical discharge among particles of the powder.
A method of producing a thermoelectric semiconductor material according to a twenty-first aspect of the invention comprises synthesizing a compound having a uniform structure represented by chemical formula (Bi1xe2x88x92xSbx)4(Te1xe2x88x92ySey)5 by applying a pulsed current to material powder for the thermoelectric semiconductor material having Bi, Sb, Te and Se mixed in a stoichiometric ratio of 4(1xe2x88x92x):4x:5(1xe2x88x92y):5y (where, 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6xxe2x89xa61to cause an electrical discharge among particles of the powder.
According to the first, fourth and seventh aspects of the invention, the thermoelectric semiconductor material is characterized by having the chemical formula AB2X4 (where, A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance or mixture of Bi and Sb (V family elements), and X is a simple substance or mixture of Te and Se (VI family elements)). In this case, spark plasma sintering device 10 is used to apply a pulsed current through powdered material 14 to cause the electrical discharge among particles of the powder to synthesize compound AB2X4 having a uniform structure. And, the compound and the sintered body may be formed at the same time. According to the present invention, the Rucklidgeite layered structure with a uniform phase can be obtained as shown in FIG. 8, so that even when the sintering is performed, the thermoelectric property can be kept at a high value because thermal conductivity xcexa becomes low and figure of merit Z becomes high than the conventional PbTe-based semiconductor material.
According to the second, fifth and eighth aspects of the invention, the thermoelectric semiconductor material is characterized by having the chemical formula Bi1xe2x88x92xSbxTe1 (where, 0xe2x89xa6xxe2x89xa61). In this case, the spark plasma sintering device 10 is used to apply a pulsed current through the powdered material 14 to cause the electrical discharge among particles of the powder to synthesize compound Bi1xe2x88x92xSbxTe1 having a uniform structure. And, the compound and the sintered body may be formed at the same time. According to the present invention, the Tsumoite layered structure with a uniform phase can be obtained as shown in FIG. 9, so that even when the sintering is performed, the thermoelectric property can be kept at a high value because thermal conductivity xcexa becomes low and figure of merit Z becomes high than the conventional PbTe-based semiconductor material.
According to the third, sixth and ninth aspects of the invention, the thermoelectric semiconductor material of the invention is characterized by having the chemical formula AYB1xe2x88x92YX1 (where, 0xe2x89xa6yxe2x89xa60.25, A is a simple substance or mixture of Pb, Sn and Ge (IV family elements), B is a simple substance or mixture of Bi and Sb (V family elements), and X is a simple substance or mixture of Te and Se (VI family elements)). In this case, the spark plasma sintering device 10 is used to apply a pulsed current through the powdered material 14 to cause the electrical discharge among particles of the powder to synthesize compound AYB1xe2x88x92YX1 having a uniform structure. And, the compound and the sintered body may be formed at the same time. According to the present invention, the Tsumoite layered structure with a uniform phase can be obtained as shown in FIG. 9, so that even when the sintering is performed, the thermoelectric property can be kept at a high value because thermal conductivity x becomes low and figure of merit Z becomes high than the conventional PbTe-based semiconductor material.
According to the sixteenth and nineteenth aspects of the invention, the Tsumoite layered structure can be obtained, so that even when the sintering is performed, the thermoelectric property can be kept at a high value because thermal conductivity xcexa becomes low and figure of merit Z becomes high than the conventional PbTe-based semiconductor material.
According to the seventeenth, eighteenth, twentieth and twenty-first aspects of the invention, the figure of merit equivalent to or higher than the PbTe-based semiconductor material can be obtained, so that the thermoelectric semiconductor material of the invention is effective as a thermoelectric semiconductor material at a middle temperature range instead of the conventional PbTe-based semiconductor material.
And, it is an object of the present invention to provide a method which can produce a compound usable as the thermoelectric semiconductor material so to have a uniform structure and also can produce it efficiently.
Therefore, a method of producing a thermoelectric semiconductor material according to a tenth aspect of the invention is characterized by having a synthesizing step for synthesizing a compound having a uniform structure by applying a pulsed current through material powder for a thermoelectric semiconductor material having a desired structure to cause an electrical discharge among the particles of the powder.
In a method-of producing a thermoelectric semiconductor material according to an eleventh aspect of the invention, the material powder for the thermoelectric semiconductor material having the desired structure according to the tenth aspect of the invention is a mixture of powder of at least two kinds of elements or alloys, and the synthesized compound is a new kind of compound different from the at least two kinds of elements or alloys.
In a method of producing a thermoelectric semiconductor material according to a twelfth aspect of the invention, the material powder for the thermoelectric semiconductor material having the desired structure according to the tenth aspect of the invention is a mixture of powder of at least two kinds of elements.
A method of producing a thermoelectric semiconductor material according to a thirteenth aspect of the invention comprises:
a step of mixing respective elements as raw materials for the thermoelectric semiconductor material so to have a desired structure and heating to melt the mixture;
a step of solidifying the heated and melted mixture of the respective elements to form an ingot material for the thermoelectric semiconductor material;
a step of pulverizing the ingot material to form powder of the ingot material;
a step of classifying the powder of the ingot material to a predetermined particle diameter or below; and
a step of synthesizing a compound having a uniform structure and also forming a sintered body of the compound by applying a pulsed current and a pressure to the powder of the ingot material having the predetermined particle diameter or below to cause an electrical discharge among particles of the powder.
A method of producing a thermoelectric semiconductor material according to a fourteenth aspect of the invention comprises:
a step of heating to melt at least two kinds of elements or alloys as raw materials for the thermoelectric semiconductor material;
a step of solidifying the respective heated and melted elements or alloys to form ingot materials of the respective elements or alloys;
a step of pulverizing the ingot materials of the respective elements or alloys to form powder of the ingot materials of the respective elements or alloys;
a step of classifying the powder of the ingot materials of the respective elements or alloys to a predetermined particle diameter or below;
a step of mixing the powder of the ingot materials of the respective elements or alloys having the predetermined particle diameter or below so to have a desired composition; and
a step of synthesizing a new kind of compound different from the at least two kinds of elements or alloys and also forming a sintered body of the new compound by applying a pulsed current and a pressure to the mixture of the powders of the ingot materials of the respective elements or alloys to cause an electrical discharges among particles of the powder to synthesize.
A method of producing a thermoelectric semiconductor material according to a fifteenth aspect of the invention comprises:
a step of pulverizing respective ingots of at least two kinds of elements as raw materials for the thermoelectric semiconductor material to form powder of the each element;
a step of classifying the powder of the each element to a predetermined particle diameter or below;
a step of mixing the powders of the respective elements having the predetermined particle diameter or below so to have a predetermined composition; and
a step of synthesizing a compound having a uniform structure and also forming a sintered body of the compound by applying a pulsed current and a pressure to the mixture of the powders of the respective elements to cause an electrical discharge among particles of the powder to synthesize.
Specifically, as shown in FIG. 3, the present invention uses spark plasma sintering device 10 to apply a pulsed current through powdered material 14. Thus, the electrical discharge is caused among particles of the powder to m synthesize compound PbBi2Te4 having a uniform structure.
More specifically, the electrical discharge is caused among the particles of the powder when the pulsed current is applied through the powdered material 14. The powder is heated owing to its own heating action resulting from the electrical discharge caused among the particles of the powder. At this time, the particles of the powder have a state of plasma among them and locally hot portions by the electrical discharge. Thus, a reaction is promoted. Defects are also caused in the crystal due to the impact of the electrical discharge, and the diffusion of atoms is promoted through the defects. Thus, the compound has a very uniform structure owing to the promoted reaction among the particles of the powder and the promoted diffusion of atoms.
The compound may be synthesized and sintered simultaneously by applying the pulsed current and a pressure to the powdered material 14 at the same time. But, it is not essential to solidify the powdered material 14 at the same time. As a procedure of the steps, the pulsed current may be applied through the powdered material 14 before applying a pressure to it. And, the powdered material 14 may be pressed before applying the pulsed current.
Here, the powdered material 14 to which the pulsed current is applied according to the invention includes the following.
(1) Mixture of at least two kinds of powdered compounds (e.g., a PbTe compound and a Bi2Te3 compound).
(2) Mixture of at least two kinds of powdered elements (e.g., Pb, Bi and Te).
(3) Powder prepared by mixing elements (e.g., Pb, Bi and Te), heating to melt the mixture, solidifying the melted mixture of the elements and pulverizing the produced ingot material.
(4) Mixture prepared by heating to melt respective compounds (e.g., a PbTe compound and a Bi2Te3 compound), solidifying the respective thermally melted compounds, forming the ingot material of each compound, pulverizing the ingot material of the each compound to prepare powder from the ingot material of the each compound, and mixing the respective powdered ingots of the compounds.
(5) Mixture prepared by pulverizing each ingot of at least two kinds of elements (e.g., Co and Sb) to form powder of each element and mixing the respective powdered elements.